.ys{
    width: 100vw;
    height: 100vh;
    background: url('https://n.sinaimg.cn/sinacn/w1366h768/20180109/574f-fyqnici9828212.jpg');
}
body{
    background: white;
    color: black;
}
.dark{
    background: #000;
    color: white;
}